7792
J . Phys. Chem. 1991, 95, 7792-7195
Methoxy Mobility and Methane Formation on the Alumina Support Alfred B. Anderson* and Sbu-Fen Jen Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: December 27, 1990)
The mobility of OCH3 and H and their reaction to form CH4 on alumina are analyzed by using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Theoretical evidence is given that OCH3, which is known to spill over from supported Ni and Pt to the alumina support, moves as an anion, OCH3-, from one AI3+ site to another, paired with a proton which moves from one 0’ site to another. The reaction of this heterolytically adsorbed pair to form CH,(g) is calculated to be more stabilizing than CH30H(g) formation. Our findings provide a possible explanation for recent experimental observations of others regarding CO methanation over Ni/AI2O3 and Pt/A1203.
Introduction At some stage, the C-0 bond is broken during methanation over metal catalysts, but it is not known when. There is convincing e~perimentall-~ and theoretical4 evidence that CO dissociates a t low temperature on some early transition metal surfaces, while on late transition metals, including Ni and pt surfaces, adsorbed CO, which binds upright through C, desorbs on heating.- Late transition metals are effective methanation catalysts when they are prepared on titania and other early transition metal oxide supports. In these cases suboxide species migrate over the metal during reduction, blocking many metal sites to CO adsorption and, at the same time, increasing methanation a ~ t i v i t y . ’ ~ ”This is called the strong metalsupport interaction (SMSI) effect. Low vibrational frequencies are observed for C O adsorbed on SMSI catalysts, and the decrease has been interpreted as catalytic activation of CO. It has been proposed that, since early transition metals are very oxophilic, the adsorbed C O binds to the metal through C and the 0 end bonds to a nearby cation of the sub~ x i d e . ’ ~ Molecular J~ orbital calculations substantiated this hypothesis and characterized the bonding to the cation as ?r donation from CO to empty d orbitals.I6 This interaction was also shown to be responsible for the nonperpendicular orientation of C O on Cr( 1 and Fe( 100) and Fe( 1 1 surfaces which is stable to dissociation at low temperatures. Not only does ?r donation weaken the C-O bonds, but the structure causes increased back-donation to the CO ?r* orbitals so that the total result is a decrease of CO vibrational frequency by several hundred wavenumbers and a weakening of the CO bond dissociation energy by an order of magnitude. For these reasons it is generally believed that dissociation of adsorbed CO is the first step in CO methanation over SMSI catalysts. A kinetic model based on this hypothesis has been developed for C O methanation over alumina-supported ( I ) (a) Shinn, N. D.; Madey, T. E. Phys. Reu. Lerr. 1984,53, 2481. (b) Shinn, N. D.; Madey, T. E. J. Chem. Phys. 1985, 83, 5928. (c) Shinn, N. D.; Trenary, M.; McClellan, M. R.; McFeely, F. R. J. Chem. fhys. 1981,75, 3 142. (2) Benndorf, C.; KrBger, B.; Thieme, F. Surf. Sci. 1985, 163, L675. (3) Zaera, F.; Kollin, E.; Gland, J. L. Phys. Rev. Leu. 1985, 121, 464. (4) (a) Mehandru, S. P.; Anderson, A. B. Sur/. Sci. 1986, 169, L281. (b) Mehandru, S. P.; Anderson, A. B. Sur/. Sci. 1988, 201, 345. (5) Steininger, H.;Lehwald, S.;Ibach, H. Sur/. Sci. 1982, 123, 264. (6) Bare, S. R.; Hofmann, P.;King, D. A. Surf. Sci. 1984, 144, 347. (7) Trenary, M.; Uram, K. J.; Bozso, F.; Yates, J. T., Jr. Sur/. Sci. 1984,
146, 269. (8) Ray, N . K.; Anderson, A. B. Surf Sci. 1982, 119, 35. (9) Allison, J. N.; Goddard, W. A., 111 Surf. Sci. 1982, 115, 553. (10) (a) Tauster, S.J.; Fung, S. C.; Garten, R. L. J . Am. Chem. Soc. 1978, 100, 170. (b) Tauster. S. J. Acc. Chem. Res. 1987,20, 389. (c) Tauster, S. J.; Fung, S.C . J . Carol. 1978, 55, 29. ( I 1 ) Rieck, J. S.;Bell, A. T. J . Card. 1985, 96, 88. (12) Vannice, M. A.; Garten, R. L. J . Card 1979, 56, 236. (13) Levin, M. E.; Williams, K. J.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. Surf.Sci. 1988, 195, 341. (14) Sachtler, W . M. H.; Ichikawa, M. J . fhys. Chem. 1986, 90, 4752. ( I S ) Zhao, Y. B.; Chung, Y.-W. J. Carol. 1987, 106, 369. (16) Anderson, A . B.;Dowd, D. Q.J. f h y s . Chem. 1987, 91, 869. I
,
,
metals.17 The alternative, that CO is hydrogenated to an intermediate prior to C-0 bond cleavage, cannot be completely excluded. Calculationsi8 have suggested that the formation of formyl, HCO(a), from CO and H chemisorbed on an iron surface will have a low activation energy and the reaction is close to energy neutral, so it might occur. It can be argued that the same interactions that weaken the C-O bond toward dissociation on metal surfaces would also stabilize the formation of a tilted HCO(a) intermediate during methane formation. Recent work has demonstrated an ability of nickelI9 and platinum20 supported on alumina to methanate C O and, at the same time, generate methoxy, CH30, bound to the alumina. Isotope labeling and vibrational analysis performed over Pt have shown that the C-0 bond has not broken: the labeled 0 in CO was found in the methoxy. The methane evolution properties of these two catalytic systems hold special interest. Two methane temperature-programmed reaction (TPR) peaks were seen over these catalysts. On Ni/AI2O3 they were around 443 and 520 K, and on Pt/A1203 they were around 493-533 and 613 K. Careful experimental analysis indicated that over Ni/A1203 the lower temperature (443 K) methane formation peak came from the hydrogenation of CO adsorbed on the metal, and during desorption of CH, at the higher temperature (540 K) peak, the support was covered by methoxy. In the absence of H2 pressure, the methoxy transferred back to Ni and decomposed to CO. On Pt/A1203 the lower temperature (493-533 K) TPR peak coincided with methoxy on the support, and its temperature was close to the higher temperature peak for Ni/AI2O3, which was also associated with methoxy on the support. The higher temperature (613 K) TPR peak was due to methanation of CO on Pt. It was concluded by the authors of both of these studies that CO, probably in a hydrogenated form, spilled over from the metal to the A1203support. Methoxy is not stable on either clean metal surface at these temperature^.^'-^^ Formate has also been observed on the support but seems to be independent of the methoxy.20 As part of the supported Pt study water was introduced to the system, and it was found to block the formation of methoxy.*O This suggested that dehydroxylation of the alumina surface is a necessary condition for its formation and that C H 3 0 is probably bound to coordinatively unsaturated (cus) surface AI3+, for it is these sites that are blocked when the surface is hydroxylated by water. There are some other pieces to the mechanistic puzzle which may fit in but by no means complete the picture. These are discussed next. (17) Baetzold, R. C. J . Phys. Chem. 1984, 88, 5583. (18) Blyholder, G.; Lawless, M. J. Am. Chem. Soc. 1989, 111, 1275. (19) (a) Glugla, P. G.: Bailey, K. M.; Falconer, J. L. J . Phys. Chem. 1988, 92, 4474. (b) Kester, K. 9.; Falconer, J. L. J . Card 1984,89, 380. (20) Robbins, J. L.; Marucchi-Soos, E. J . Phys. Chem. 1989, 93, 2885. (21) Sexton, B. A. Sur/. Sei. 1981, 102, 271. (22) Demuth, J. E.;Ibach, H. Chem. fhys. Lerr. 1979, 60, 395. (23) Gibson, K. D.; Dubois, L. H.Sur/. Sci. 1990, 233, 59.
0 1991 American Chemical Society
OCH3 Mobility and CH4 Formation on Alumina When adsorbed on Ni-AI alloy surfaces, CO shows a vibrational frequency decrease24 which we have explained occurs by a mechanism similar to that in SMSI systems: CO bridges Ni and AI centers with A donation through 0 to empty valence p orbitals on This leads to bond stretching and weakening. A recent study of CO methanation on supported A1203 has found that whether or not activity enhancement occurred in reduction experiments at 498 K depended on how the metalsupport system was prepared.% It was pointed out that A1203can dissolve in acidic solution, and preparation of supported Pt from H,PtCI, resulted in an acidic solution that probably dissolved some of the support and deposited it in some form on the metal and back on the support. This system gave enhanced activity but preparation using [Pt(NH3),]CI2 did not, apparently because none of the support dissolved in the nonacidic solution. Reduction of Fe(N03)3 over A1203also deposited support material on the metaL2' Interestingly, in the studies discussed above, where methoxy was produced on the support, the preparations involved H2PtC16and Ni(N03)3, so in both cases activity may have been influenced by support material dispersed on the Pt and Ni. It was proposed that the support material may be reduced to AIO, during the reducing conditions (high temperature and H, pressure) used during CO methanation.m Thus, it might be activating CO by the mechanism mentioned above, involving A donation to AI. The question now arises concerning whether the observed CH30 sites are on the support or on the AIO, that is dispersed over the metal. Since the amount of adsorbed CO is hardly decreased when AIO, is present (unlike SMSI catalysts involving early transition metal oxides10a*bv'6), the large observed quantities of CH30 must be bound to the support. The role (if any) of AIO, on the metals in causing C H 3 0 to form on the support is unknown, but it seems almost certainly responsible for increased methanation activity. The temperature chosen for the reduction experiments in ref 26 must also be considered in this regard. Its value, 498 K, is in the A1203TPR peak and in the low-temperature tail of the Pt TPR peak shown in ref 20. The rate constant for CH4 formation from CO pulses in H2 at 498 K was seen to increase as a function of increased catalyst reduction temperature, and O2treatment reduced the rate. This is consistent with reduced AIO, species on the Pt activating the CO for methanation over the Pt surface. It was noted that on earlier transition metals, which already have good CO methanation activity, Rh and Ru, AIO, did not provide further enhancement. From the above, it may be concluded that at least some catalysts active for C O methanation have C O initially activated by A donation bonding of the type M-C-0-M or M-C-O-M"+. Whether the first step is CO dissociation or hydrogenation to formyl, CHO, or other intermediates, followed by CO dissociation is not known. Infrared studies have shown no HCO on the surface of Ni/A1203 during CO methanation,28 but such an intermediate would be undetectable if it dissociated immediately to adsorbed C H and 0. The fragments C, CH, CH2, and CH3 have been detected by static secondary ion mass spectroscopy (SIMS) following C O methanation over Ni( 1 1 but in this case no A1203was present. Hydrogenation of formyl to formaldehyde appears unlikely over the clean unsupported catalysts because at methanation temperatures formaldehyde decomposes to adsorbed CO and H.30*31 Methoxy also decomposes to adsorbed CO and H on these metal^.^'-^^ However, over saturated surfaces32 and
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1193
over surfaces with preadsorbed oxygen32b.33 methoxy can be stab i l i ~ e d ,and ~ ~ formyl and formaldehyde have been observed spectroscopically at low temperature^.^^ Therefore, one cannot completely exclude mechanisms involving those intermediates. Methoxy is intriguing. The supported nickel results show its methanation at a higher temperature than CO methanation over the Ni surface. It might be supposed that at the higher temperature the CH30 spills back to Ni, decomposes, and hydrogenates. Yet on supported Pt the lower temperature TPR methane peak corresponds to the consumption of CH30, and methane is evolved from CO on the Pt at -100 K higher temperature. Therefore, it appears that in both cases, since the TPR peak occurs in the same temperature range for the C H 3 0 associated CH4 evolution, CH30hydrogenation must be taking place on the A1203 or at its interface with the supported metal particles. We note that C H 3 0 forms on the support at 1385 K on both the Ni/A1203 and Pt/AI2O3 systems, well below the temperature of the TPR CH4 peaks. What is one of the most important still missing pieces of the mechanistic puzzle is an experimental investigation of whether C H 3 0 forms on A1203 in the absence of AIO, on the metal particles. It then would form either if CO can be hydrogenated by H that is spilled over onto the A1203, and such spillover is known for this o ~ i d e , 'or ~ *if ~C~H 3 0 formation and its spillover can be activated at the metal-support interface. We have undertaken a theoretical study of methoxy stability and its mobility and reactivity over A1203in order to gain further understanding of the support effect. The atom superposition and electron delocalization molecular orbital (ASED-MO) theory is used in conjunction with cluster models.
Method and Models The semiempirical ASED-MO theory employs atomic parameters for predicting properties of molecules.34 It is based on the physical model of partitioning molecular charge density distribution functions into free atom components and the charge redistribution component due to bonding in the molecule. The integral of the Hellmann-Feynman electrostatic force on a nucleus as its corresponding atom is brought from a noninteracting distance into a molecular conformation yields two energy components with this charge density partitioning. One, due to atom superposition, is repulsive and is denoted by ER, and the other, due to the electron delocalization redistribution density, is attractive and is denoted by ED: E = ER + ED (1)
In a polyatomic molecule, ER is a sum of pairwise repulsion energies and ED is a sum over half the nuclei. Only ER is easily calculated. To find ED by use of the Hellmann-Feynman force theorem as described above would require the solution of the Schriidinger equation to find the charge density redistribution function, a difficult task which, if could be accomplished, would make this model superfluous. The electron delocalization energy calculated by using a modified extended Huckel Hamiltonian has been found to approximate E D quite well for many molecules E = ER + AEMo (2) where AEMo= Cn,cIMo- CCn,c,O i
(24) Chen, J. G.; Crowell, J. E.; Ng, L.; Basu, P.; Yates, J. T., Jr. J . Phys. Chem. 1988, 92, 2574. (25) Anderson, A . B.; Jen, S.-F. J . Phys. Chem. 1990, 94, 1607. (26) Taniguchi, S.; Mori, Y . ; Hattori, T.; Murakami, Y . J . Chem. SOC., Faraday Tram. I 1989, 85, 3135. (27) Perrichon, V.; Charcosset, H.; Barrault, J.; Forguy, C. Appl. Cuful. 1983, 7, 21.
a i
(3)
In eq 3 the n. are molecular orbital or atomic orbital occupation numbers, cIMb is the energy of molecular orbital i. a is the atom index, and cI(1 is the energy of atomic orbital i for atom a. With the approximations of eqs 2 and 3, better results are usually obtained when each contribution to E , is calculated by using the charge density function of the more electronegative atom and the
(28) Stockwell, D. M.; Chung, S.J.; Bennett, C . 0. J . Cum/. 1988, 112, 115
(29) Kaminsky, 1986, 108, 1315.
M.P.; Winograd, N.; Geoffroy, G. L. J. Am. Chem. Soc.
(30) Abbas, N.; Madix, R. J. Appl. Sur/. Sci. 1981, 7, 241. (31) Henderson, M. A.; Mitchell, G. E.; White, J. M. Sur/. Sci. 1987, 188, 206.
(32) (a) Anton, A . B.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. SOC.1985. 107, 5558. (b) Anton, A . B.; Parameter, J. E.; Weinberg, W. H. J . Am. Chem. SOC.1986, 108, 1823. (33) Solymosi, F.; Tarnkzi, T. 1.; Berkd, A . J . Phys. Chem. 1984, 88, 6170. (34) Anderson, A . B. J . Chem. Phys. 197562, 1187.
1194 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1-fold cus AI3+
Anderson and Jen
3-fold cus At3+ I
I
P
n
Figure 1. AllOs cluster model of I-fold coordinatively unsaturated surface AI3+ cation and Al2O6model of a 3-fold coordinatively unsaturated surface cation. TABLE I: Parameters Used in the Calculations: Principal Quantum Numbers, n, Ionization Potentials, IP (ev), Slater Orbital Exponents, f (nu)
P
S
atom
n
IP
t
n
IP
t
AI3+
3 2
C
2
15.59
0
2 1
27.48 12.6
1.5213 1.7458 1.608 2.246 1.2
3
0"
12.62 26.48
7.986 11.62 10.26 12.62
1.5041 1.7266 1.568 2.221
H
Figure 2. Calculated structure for OCH3 bound to the 3-fold coordinatively unsaturated cation of Figure 1.
2
2 2
Coulombic interaction of the other nucleus in the pair with it. This is done here. Slater-type atomic orbitals are used, and the modified extended Huckel Hamiltonian depends on measured valence orbital ionization potentials. In molecules where the initial charge transfer is excessive and bonds are too short with these parameters, self-consistency is emulated by decreasing the anion orbital exponents and ionization potentials and increasing them for the cation. Rules for this and a discussion have been published.35 Parameters used for this study are given in Table I. For AI and 0, parameters are those used in a past study of the bonding at interfaces between alumina and nickel surfaces.36 The C and 0 parameters are taken from the previous study of CO adsorption on Ni( 1 11) and its interaction with an adsorbed AI atom.2s The H ionization potential is treated in the same way as those for C and 0. The alumina support is generally considered to be yA1203in the experimental studies. The detailed atomic structure of -pA1203 has not yet been satisfactorily established. Fourier transform infrared, thermogravimetric analysis, and X-ray photoelectron spectroscopic studies3' have indicated that aluminas prepared under different conditions will have different types of cus AI3+ surface sites after dehydrogenation at various temperature. In the absence of detailed surface structure information we have chosen a small cluster model based on the known structure of a-A1203and have focused on its cus AI3+ cation sites. Figure 1 shows an AI2O6+cluster which has a surface AI3+ in 3-fold coordination (and also one in 4-fold coordination). A second cluster model AI2O8Ihis also shown. It has a surface cus AI3+ in 5-fold coordination, and the other one is in full (6-fold) coordination. Atom position parameters for a-A1203are taken from the litera t ~ r with e ~ ~AI-0 bond lengths of 1.888 1 and 1.9275 A. Mobility of Methoxy on A1203. Surface defect sites are likely to have surface AI3+ in low coordination, and binding at such a site is discussed first. The binding of OCH3on the 3-fold cus AI3+ site is calculated to be strong, 3.7 eV. The structure is in Figure 2. Two components contribute to this strength: the covalent stabilization of the u lone-pair orbital by the higher lying band gap dangling AI3+ orbital and the electron transfer from a neighboring surface 02-to the *-type radical orbital, formally generating a methoxy anion. As shown in Figure 3, the electron transfer oxidizes a neighboring 02- to 0-. Methoxy radicals when isolated on the alumina surface might be expected to be immobile because of the strong bonding to cation sites isolated from one another by anions and unabte to by themselves migrate beyond (35) Anderson, A. B.; Grimes, R. W.; Hong, S.Y . J . Phys. Chem. 1987, 91, 4245. ( 3 6 ) Anderson, A. B.: Mehandru, S.P.; Smialek, J. L. J . Electrochem. Soc.
1985. 132, 1695. (37) Datta, A. J . Phys. Chem. 1989, 93, 7053. (38) Megaw, H. D. Crystal Structures: A Working Approach; W. B. Saunders: Philadelphia, 1973; p 226.
-30 Figure 3. Molecular orbital correlation diagram for OCH,' binding to a 3-fold cus AI3+on the AI,O, cluster model.
the interface region. In fact, the calculations predict a weak repulsion for OCH3 adsorption on a surface 02-despite the formation of an occupied 0-0 u bonding orbital. The repulsion is caused by the promotion of an electron into the 0-0 u* antibonding counterpart orbital. The calculations indicate weaker binding of OCH; to the cation site, 2.0 eV. Might it be that the methoxy anion can migrate across the surface while experiencing electrostatic stabilization from the diffuse positive hole charge it created on the oxygen anions? Our calculations cannot answer this question, but it seems unlikely. The 0- that the calculations predict is formed when OCH, is bound to a cation on the surface will attract H atoms spilling over from the metal. The calculated 0-H bond strength in this case is 4.1 eV, which is considerably stronger than the binding energy of a hydrogen atom to a Ni or Pt surface (which is about 2.7 eV39*40). From this it can be concluded that H+-02- and CH30--A13+ ion pairs will be stable on the alumina surface, and a mechanism for their migration over the surface can be considered. It is probable that the ion pair would migrate from site to site more easily than OCH3 by itself, and if the proton mobility is high the proton might be able to provide a stabilizing interaction for the methoxy anion as it moves. It therefore seems likely that the heterolytic pair migrates over the surface, H+ from 02-to 02-and CH30- from AI3+ to AI3+. The mobility of spilled-over hydrogen has been demonstrated by its rapid rate of exchange with surface hydroxyl hydrogen on alumina supports4' and its measured activation energy for surface diffusion of 28.5 kcal/m01.~~However, whether the mechanism involves a coordinated motion of heterolytic H+-0*- and H--A13+ pairs is not established experimentally. The pairs would have to remain in close association, though their ions could exchange with other nearby pairs, so that the H+.-OCH3- or H+-H- throughspace Coulombic interaction keeps them stable. We note that migration barriers between the surface cations that have higher than 3-fold coordination should be relatively small. This is because the adsorption energies calculated for OCH3 (39) Christmann, K.; Schober, 0.;Ertl, C . :Neumann, M. J . Chem. Phys. 1974.60. 4528. (40) Poelsema, B.: Mechterscheimer, G.; Comsa, G . Surf. Sci. 1981, 1 1 1 , 519. (41) Cavanaugh, R. R.; Yates, J . T., Jr. J . Catal. 1981, 68, 22. (42) Kramer, R.; Andre, M.J . Catol. 1987, 58, 287.
J. Phys. Chem. 1991, 95, 1195-7804 and OCH3-are weaker, 1.7 and 0.07 eV, respectively, for 5-fold coordinate AI3+. These ideas also apply to H migration across A1203surfaces. The surface capacity of alumina for hydrogen is about IO-' times the number of possible O H groups,42 which suggests that most of the chemisorbed hydrogen occupies defect sites and will diffuse rapidly until finding such sites, which is also what our calculated chemisorption energies would indicate. Decomposition of the Heterolytic Pair: Methane vs Methanol Formation. Since the heterolytic pair is so stable, methanol generation from it would be a high-temperature process, and indeed none has been observed experimentally. Related to this is the strongly held dissociative chemisorption phase of methanol which has been observed on activated (partially dehydroxylated) 6-A1203.43 From microcalorimetry, the heat of adsorption was estimated to be -2.3 eV/CH30H in the low-coverage limit. The adsorption was thought to be heterolytic, with CH,O- bound to AI3+of low coordination. The reverse reaction, which corresponds to the recombination of the heterolytic pairs and the desorption of C H 3 0 H , would then be endothermic by 2.3 eV. Our calculations yield 2.12 eV for this process when CH30- is bound to 3-fold coordinated AI3+. Recombination to form CH4 and an 0 atom on the A13+ is endothermic by only 0.65 eV according to our calculations, making this a likely process. The 0 atom + 02or the two 0-which are formed by the elimination of methane would react quickly with gas-phase or spilled-over hydrogen to form surface OH or water. If H2 migrates over the surface by a heterolytic H+-02- + H--AI3+ pair mechanism, the reaction with 0 would yield two OH- and a cus AI3+.These different types of OH, those associated with H- and those not, should be dis(43) Busca, G.; Rossi, P. F.; Lorenzelli, V.; Benaissa. M.;Travert, J.; Lavalley, J.-C. J. Phys. Chem. 1985, 89, 5433.
7795
tinguishable by vibrational measurements. Such measurements might be helpful in determining whether the hydrogen that spills over diffuses over the A1203surface in the heterolytic form. The energy barrier for CH4formation cannot be calculated with the present A1203cluster model. The reaction might be expected to be of the SN2type, with H- from OH- on the surface displacing 02-from the adsorbed methoxy anion. The activation energy for OH- displacement from ROH by H-in the gas phase has been calculated to be about 21 kcal/moL4 To model this process would require a large cluster with a "catalytic pocket" where O H would be directed toward CH30- in such a way that a four-centered O-H..CH,-O transition state could form. Such sites would not be characteristic of a smooth surface but would be associated with steps or defects. A mechanism wherein CH; is released, subsequently capturing H to form CH4, would require weakening of the methoxy C-0 bond. This would happen if the surface of the alumina is reduced by, say, reductively homolytically adsorbed hydrogen so that electrons occupy AI band gap surface dangling orbitals. The reasons were given in a recent p u b l i c a t i ~ nand ~ ~ also account for the weakness of the 02--methoxy bond discussed above: when a radical binds to 02-, the electron promotion energy in the u* orbital renders the bond weak. Acknowledgment. S.-F. Jen is grateful for a graduate Fellowship from the BF Goodrich Co. Registry No. Carbon monoxide, 630-08-0; methoxy, 2143-68-2; methoxide, 33 15-60-4; hydrogen ion, 12408-02-5; methane, 74-82-8; methanol, 67-56-1; alumina, 1344-28-1. (44) Shi, Z.; Boyd, R. J. J. Am. Chem. SOC.1990, 112, 6789. (45) Shiller, P.; Anderson, A. B. J . Phys. Chem. 1991, 95, 1396.
Primary and Secondary Reaction Pathways in Ruthenium-Catalyzed Hydrocarbon Synthesis Rostam J. Madon,+ Sebastian C. Reyes, and Enrique Iglesia* Corporate Research Laboratories, Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801 (Received: January 18, 1991)
Residence time studies show that n-paraffins, a-olefins, and cis-2-olefins are primary products during hyL. ocarbon syn . A s on Ru catalysts. Their formation, as well as that of branched isomers, is consistent with previously proposed surface reactions of alkyl groups on metal surfaces. Secondary hydrogenation and hydrogenolysis of a-olefins are inhibited by the water product of the synthesis step. However, a-olefin readsorption and surface chain initiation and cis-to-trans isomerization take place as secondary reactions. The decrease in a-olefin selectivity with increasing CO conversion and molecular size reflects the greater extent of readsorption as bed and pore residence times of a-olefins increase. Readsorption of a-olefins and chain initiation increases with molecular size because the rate of removal of olefins from liquid-filled catalyst pores is decreased due to intraparticle diffusion limitations. This diffusion-enhanced olefin readsorption accounts for the observed deviations of carbon number distributions from those predicted by Flory polymerization kinetics. Chain growth probability increases with chain length until an asymptotic value is reached. Readsorption decreases the contribution of termination via hydrogen abstraction to chain growth kinetics and leads to a heavier, more paraffinic product. In effect, differences in selectivity between small and large hydrocarbons are due to the increasing influence of pore residence times and the decreasing influence of bed residence times on secondary reactions as olefin size increases.
Introduction It became a p arent during early studies of Fischer-Tropsch (IT)synthesis', that olefins, paraffins, and oxygenates were all part of the product spectrum. Herrington' first suggested that growing hydrocarbon chains on catalyst surfaces terminate as
P
'Current address: Engelhard Corporation, 101 Wood Ave., Iselin, NJ 08830.
*To whom correspondence should be addressed.
paraffins or olefins and that the latter readsorb and initiate new growing chains. Later, Friedel and A n d e r ~ o nusing , ~ thermodynamic arguments, concluded that a-olefins and oxygenates were the major primary products. Pichler et al.,5-6on the basis of bed (I) (2) (3) (4) (5)
Fischer, F.; Tropsch, H.Ges. Abh. Kennr. Kohle 1928, 10, 3 13. Pichler, H.Ado. Cafal. 1952. 4 , 271. Herrington, E. F. G.Chem. Ind. 1946. 347. Friedel, R. A.; Anderson, R. B. J . Am. Chem. SOC.1950, 72, 1212. Pichler, H.; Schulz, H.; Elstner, M. Brenstoff-Chem.1967, 48, 78.
0022-365419112095-7795%02.50/0 0 1991 American Chemical Society
